Choice of materials of construction for mechanically demanding high temperature applications--particularly under reactive environments, such as oxidizing environments--is very limited. Lack of such materials, for example, imposes limits on the performance of turbine engines for both power generation and aircraft propulsion. In turbine engines for aircraft, a high output to weight ratio is desired. Engine efficiency increases with increasing temperatures in the combustion section. The temperature limiting factor in this application is availability of materials of construction for turbine airfoils. These are presently made of nickel-based superalloys, but metals technology is approaching the upper temperature limit, and new materials of construction are needed to provide further advances. Ceramics, especially oxide ceramics have been suggested for this application, because of theoretical big strength and oxidation resistance. However, lack of mechanical durability and strength in actual application has prevented their use in this demanding application. These deficiencies could be overcome by incorporating reinforcing fibers into the ceramic body, to provide a ceramic matrix composite. Unfortunately, suitable reinforcing fibers have heretofore not been available.
This invention provides surface strengthened single crystal oxide fibers, particularly aluminum garnet fibers, which can be used for reinforcement in ceramic and metal matrix composites, which are suitable for use in high temperature environment, including oxidizing environments. These fibers are strengthened and protected against environmental attack and mechanical damage by provision of an epitaxial compressive surface coating, which puts the surface of the fiber under significant compression.
Compressive surface layers are employed widely to improve the low temperature strength and impact resistance of brittle solids and objects. A common example is "tempered" glass for automotive and architectural applications. Surface compression is achieved by a variety of methods including heat treatment, shot peening and ion exchange. The stresses produced by these methods generally relax upon exposure of the solid to temperatures in excess of about 0.5 times the absolute melting point.
Compressive epitaxially deposited layers have previously been provided on single crystal laser media, including on single crystal yttrium aluminum garnet laser rods for improvement of low temperature strength and durability (U.S. Statutory Invention Registration H557 by Morris et al. for "Epitaxial Strengthening of Crystals"; Marion et al., Compressive epitactic layers on single-crystal components for improved mechanical durability and strength, J. Appl. Phys. 62, 2065-2069 (1987)).